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Initial Fault identification

Gas Fired Central Heating Systems   problems and diagnosis

 This two-part article described the two most common forms of gas fired central heating installations encountered in UK housing.

 This second part examines some of the most commonly encountered and reported faults for these systems. It provides some diagnostic options that can be considered prior to calling out qualified and experienced professionals.

 By Nigel Dann -Senior Lecturer and Researcher at the Faculty of the Built Environment, University of the West of England, Bristol

 Boilers - in brief

In the previous article we explained the two most commonly encountered gas fired central heating systems in the UK. These are:

The traditional vented boiler: This is fed by relatively low pressure water from a high level, small and indirect tank which also allows for the safe removal of any excessive water expansion. The boiler heats a primary circuit which creates a store of hot water, via an indirect hot water cylinder, and it also provides hot water to a series of radiators. The installation will also include a range of separate controls, including a motorised valve, a pump, a programmer and a range of thermostats.

The combi system: This mains pressure fed boiler provides both space heating and instantaneous hot water; hence there is no requirement for either cold or hot water storage tanks or cylinders. The boiler package contains integral controls such as thermostats, pump and motorised valves. In addition, because of the extra pressure as it is mains fed, the boiler package also includes lots of safety features.

Problems with the boiler and possible causes

1.  Loss of pressure (less than 1 bar): Combi boilersare generally designed to operate at a minimum pressure of 1 bar and a maximum pressure of 2.5 - 3 bar. If the pressure is higher or lower than this, the boiler will not operate. A dial on the boiler will provide a read-out of the pressure in the boiler when it is running. Common causes for pressure loss are leakage somewhere in the system – check pipe work, and in particular any compression fittings, the radiators, and in particular their valves, and around the boiler itself. The excessive removal of water from bleeding radiators can also lead to a pressure drop. The system needs topping up via the filling loop at the boiler, once the cause has been identified and remedied.

2.  Excessive pressure: (more than 2.5 bar).Excessive pressure in combi boilers will cause the Pressure Reduction Valve, a safety feature in the boiler, to open and the boiler to cut out. Common causes are the filling loop remaining partially open or a faulty/damaged pressure vessel (another safety feature in the boiler). Specialist attention will be required to diagnose and treat these problems.

3.  Thermostats set too low: Check that the room and boiler thermostats are set appropriately. A consideration of the location of the room thermostat is important – is it located in a particularly warm/cold/draughty spot? Thermostats should also be periodically checked to make sure they are functioning correctly.

4.  Is the pilot light working / does the boiler fire? Earlier boilers will have pilot lights, more recent boilers have electronic ignition. A thermocouple is a safety device which cuts the gas supply if a particular temperature is not achieved. This can become faulty and will stop the boiler from firing.

5.  Is the programmer functioning properly? Does the programmer have electrical power? Are the time and programme functions appropriately set?

6.  Are the mains gas and electrical supplies to the heating system still connected? Clearly, if either are not, then the boiler and system will not work.

Other boiler related issues

The gas-air mix is critical for efficient and safe boiler operation. The colour of the flame in the boiler should be blue/purple. If there are any signs of a yellow flame this can indicate an unclean, inefficient burn which will leave deposits in the combustion chamber and the flue – a potentially dangerous condition.

A noisy boiler can be caused by a build up of sludge in the system or excessive air. Professional verification of the cause of such problems is required.

Carbon Monoxide poisoning is fatal and is a potential risk with all gas burning domestic appliances. There is a legal requirement for an annual safety check of gas burning appliances and flues in all rented properties. The inspection must be carried out by a Gas Safe Register gas installer/technician. It is also wise to include carbon monoxide detectors at key locations in any property with any gas installations. It is important to note that these cannot be regarded as a reason not to carry out regular checks and maintenance.

Problems with radiators and possible causes

1.  None of the radiators are heating up: Firstly, this could be that the water pump (often located near the boiler) is not effectively functioning. Check the function by feeling for vibration in the pump. Secondly, it could relate to air locks within the pipe work. This is resolvable by bleeding the pump (if it has a bleed valve and/or it’s accessible). Remember to top up a combi system via the filling loop after bleeding the system - this should be automatic in the vented system.

2.  Some of the radiators are not heating up: This can occur in systems which are zoned and here the programmer may not be working. Air locks, in part of the pipe work system, could also create this problem. A third possibility is that the radiator system needs rebalancing, reducing the rate of the hot water flow from the boiler to radiators nearer the pump and increasing the hot flow to radiators further away from the pump.

3.  Radiators on the ground floor are working, but not on upper floors: In vented systems this can occur if the feed and expansion tank is faulty and not topping up the primary circuit.

4.  Radiators on upper floors are working, but not on the ground floor: This is usually caused by a faulty pump.

5.  Issues with individual radiators:

a. An individual radiator not heating up can be caused by individual Thermostatic Radiator Valves (TRVs) which can control the heat output of that specific radiator. These can become stuck, particularly after summer periods. It could also be that the valve is set too low. The purpose and settings of the TRVs can be difficult to understand by some occupiers. The provision of technical data, or even better, an explanation of the whole system, and specifically, the controls, including the TRVs, can forestall a number of occupier enquiries and mis-reported faults.

The more conventional ‘Lockshield’ radiator valves can also become stuck in a particular position, though nowadays these are less commonly encountered.

b. If an individual radiator is simultaneously cold at the top and warm at the bottom then it is likely that there is an airlock. This can be resolved by bleeding the individual radiator.

c. A cold patch at the middle/bottom of an individual radiator can be caused by a build up of sludge and rust. Flushing this out is important and the specific action depends on what system is installed. On a combi system the individual radiator will need to be removed for individual flushing prior to being replaced. This can also be done on vented systems; however, with this system, it is possible to de-sludge the whole system by adding a proprietary de-sludging compound to the feed and expansion tank and flushing, draining and refilling. The new water introduced to the primary circuit should be treated with anti-scaling chemicals.

Inactive periods

A number of the above, together with other potential problems can occur to central heating installations because they are frequently inactive during the summer months. Common problems which occur are:

Air entering the radiator and pipe work;

Feed and expansion tanks, despite theoretically having lids, are frequently encountered without them. Left without a lid, over a hot summer in the roof void, the water they contain can be subject to evaporation. This should not be a problem if the ball float valve is operative. However, this maintenance hungry valve requires regular intervention and frequent failure/faults are not uncommon. If it has failed, the feed and expansion tank can be left without water or the means to automatically fill this up;

Pumps and radiator valves can become stuck whilst inoperative for an extended summer period.

General advice

It is wise to undertake all annual safety checks at the same time as the system’s annual service and maintenance checks. The clear message here is that it is ideal to undertake all of these at the end of the summer, typically just prior to the commencement of the heating season.

It should also be remembered that there are various insurance schemes which provide both the occupier and landlord with a variety of levels of repair, maintenance, replacement and servicing for central heating systems. The cost of the premiums needs to be balanced with a number of considerations, such as the age of the installation, any restrictions the scheme imposes and the peace of mind such insurance provides.

All of the issues considered here must not be seen as a substitute for experienced, qualified professional advice, inspection and, most importantly, regular servicing and maintenance. Combi systems in particular, because of their highly engineered nature and the additional safety features, require more frequent intervention than most vented systems.

Further information:

www.gassaferegister.co.uk

www.nationalgrid.com/uk/Gas/

www.which.co.uk/advice/home-heating-systems/gas-central-heating/index.jsp

Standard Wall Mounted Boiler

Combi Boiler

Floor Mounted Boiler

Boiler Faults

Leaks

Pumps & valves

Radiators